This invention relates to open coil heat treatment of strip metal and more particularly to a process for preventing undesirable distortion of an open coil during heat treatment.
The invention will be disclosed hereinafter in connection with the heat treatment of open coils of light gauge steel strip since it provides exceptional advantages in such use, although it may be used in the heat treatment of other strip material in open coils.
In the open coil heat treatment of such strip metal, a tight wound coil of strip metal is first recoiled into an open coil in which the spiral convolutions of strip metal in the coil are spaced apart. Such an open coil is then supported with its axis vertical on an open grid-like base structure and a suitable heated gas is caused to flow vertically either upwardly or downwardly outside and inside of the coil through the spaces between the convolutions of the coil to contact both sides of the metal strip and thus effectively heat the metal, as in the known process and apparatus disclosed in Wilson and Corns U.S. Pat. No. 3,114,539.
In heat treatment such as annealing, the steel strip in the coil may be heated to as high as 1750.degree.F. by passing a suitable heated gas through the spaces between the convolutions of the coil. Other open coil treatments may involve modifying the chemical composition of the strip metal. For example, the carbon content of steel strip may be modified by the use of a suitable gas. Furthermore, treatments may involve oxidation, bright annealing, gas alloying and treatments involving the application to the surfaces of the strip metal of materials that will react with the constituents of the metal and the treatment of the surfaces by passing a suitable heated atmosphere between the spaced strip convolutions.
Since in open coil heat treatment, the lower edge portions of the convolutions of the open coil rest on a supporting grid or base, they satisfactorily maintain their original positions during heat treatment and are not harmfully disturbed by localized distortion of the metal due to thermal expansion or by the blast of heated atmosphere passing through the convolutions of the coil. However, when heat is applied by forcing heated atmosphere downwardly through the spaces between the convolutions from the top to the bottom of the open coil, the unsupported top edge portions of the spaced convolutions may flutter and move out of position due to the action of the high velocity stream of hot gas as it is forced down into the spaces between the coil convolutions. This may result in a permanent distortion of the upper edge portion of the strip which is difficult to remove and interferes with the proper recoiling of the heat treated opened coil back into tight wound form, and also with desired usage of the strip material.
Particular problems occur when the strip is of very light gauge, as from about 0.003 to 0.035 inch thick. In the absence of the present invention, strip steel of such small thickness is particularly susceptible to distortion at the upper end of the coil for the reasons indicated above, and because radiant heat transferred to the upper edge portions of the strip in the coil from the heated enclosure structure that is normally used around the coil in the heat treatment causes localized heating and thermal distortion of such edge portions of the coil.
Such permanent distortions of the edge portions of the coil are commonly referred to as "paneling" and cause the convolutions to contact each other so that all parts of the strip do not receive uniform treatment, cause difficulties in rewinding the strip into a tight coil, and reduce the quality of the product so it cannot be used for the desired purposes, as for example, for tinplate use.